1. Field of the Invention
The present invention relates to an optical information processor for optically reading information from a storage medium or writing information on a storage medium.
2. Description of the Related Art
Recently, various types of optical information processors for reading and writing information optically have become more and more popular. Among other things, optical disk drives for reading and writing data, representing various types of information including video, image and audio, from/on a disk storage medium such as a Compact Disc (CD), a Mini Disk (MD), a Digital Versatile Disc (DVD) and a Blu-ray Disc (BD), have become particularly popular among consumers.
In each of these drives, a semiconductor laser is usually used as a light source to perform read and write operations. The light that has been radiated from the semiconductor laser is converged by a lens or any other optical element on a storage medium. In writing data on the storage medium, the optical power of the semiconductor laser is set relatively high, thereby increasing the intensity of the light that makes a beam spot on the storage medium and changing a physical property (such as the reflectance or magnetic property) of the storage medium. In this manner, the data is written as marks or pits on the storage medium. In reading data from the storage medium, on the other hand, the optical power of the semiconductor laser is set lower than in the write operation, thereby sensing a variation in the physical property (such as the reflectance or magnetic property) of the storage medium at the marks or pits and reading the data from the storage medium.
Thus, such a drive needs to change the optical power of its light source in a broad range from the low power to the high power. Also, the best light intensity of the beam spot to write data with good stability changes from one drive to another according to the rate of scanning done on the storage medium or the type of the given storage medium. For these reasons, in performing a write operation, the optical power of the light source needs to be controlled so as to keep the best writing conditions always.
Meanwhile, there are increasing demands for high-speed data writing these days. To write data on the same type of storage medium faster, a technique of increasing the rate of scanning the storage medium is usually adopted. However, as the scanning rate is increased, the energy of the radiated light applied to a unit area of the storage medium decreases. That is why to ensure the minimum required light intensity for writing, the optical power of the light source must be further increased.
On the other hand, various techniques of increasing the storage densities have also been researched thoroughly nowadays. As a result, the physical dimensions of marks or pits to be left on a storage medium have become smaller and smaller. To make (or record) those small marks on a storage medium just as intended, not only the amount of time in which the storage medium is irradiated with the light but also the intensity of the radiated light need to be controlled with high precision.
Thus, in order to control the optical power of the light source highly precisely in a broad range, an optical disk drive detects the intensity of the light emitted from the light source and controls the optical power of the light source based on the result of the detection. More specifically, the optical power of the light source is monitored in real time by getting a portion of the emission of the light source detected by a detector, thereby controlling the optical output of the light source (see Japanese Patent Publication No. 2907759, for example).
This conventional technique will be outlined with reference to FIGS. 22A through 26. FIGS. 22A and 22B are respectively a top view and a side view illustrating the arrangement of an optical system in a conventional optical disk drive. The top view illustrated in FIG. 22A shows the optical system as viewed from over the storage medium. As shown in FIGS. 22A and 22B, light 222 emitted from a semiconductor laser 221 as a light source is transformed into a parallel beam by a collimator lens 223. A portion of the parallel beam is transmitted through a reflective mirror 224 and incident onto a detector 225 for monitoring the optical power. The output of the detector 225 is supplied to a laser controller 226, which adjusts the output of the semiconductor laser 221 to a required value. On the other hand, another part of the parallel beam is reflected by the reflective mirror 224, transmitted through a condenser lens 227 and then converged toward a storage medium 228. The light that has been reflected from the storage medium 228 follows the same path in the opposite direction, and is diffracted by a detecting diffraction element 229, transmitted through the collimator lens 223, and then incident onto signal detectors 2210 and 2211, which are arranged near the semiconductor laser 221. Various types of signals including focus error, tracking error and RF signals are detected from the light that has entered the detectors 2210 and 2211. The configurations of the detectors 2210 and 2211 and the methods of detecting those various signals are not essential features to be compared to the present invention and already well known in the art, and the description thereof will be omitted herein.
FIG. 23 shows the far-field pattern (FFP) of the light that has been radiated from the semiconductor laser 221. In the far-field pattern 231 shown in FIG. 23, the center of the aperture area 232 (which will be sometimes referred to herein as “Area A”) defined by the condenser lens 227 and the center of the photosensitive area 233 (which will be sometimes referred to herein as “Area B”) defined by the optical power monitoring detector 225 substantially agree with that of the far-field pattern 231. As shown in FIG. 24, the far-field pattern 231 has an almost normal distribution. If the condenser lens and optical power monitoring detector are arranged such that the center of the aperture area and the center of the photosensitive area agree with that of the far-field pattern, then the light emitted from the light source can be used most efficiently. That is why the semiconductor laser 221, condenser lens 227 and detector 225 are normally arranged such that the respective centers of the aperture and photosensitive areas 232 and 233 agree with that of the far-field pattern 231.
Recently, a semiconductor laser called a “real refractive index guided laser”, of which the operating current is reduced for the purpose of increasing its optical power and improving its performance at elevated temperatures, has been used actually (see Matsushita Technical Journal Vol. 45, No. 6 (December 1999), for example). This semiconductor laser is characterized in that the angle of radiation of its emission in the horizontal direction generally changes according to its optical power. FIG. 25 schematically shows how the intensity of the light emitted from a real refractive index guided laser changes with the angle of radiation of the light in the horizontal direction. In the example shown in FIG. 25, the higher the intensity of the light, the wider the angle of radiation of the light.
If a real refractive index guided laser having such a characteristic is used as the light source of an optical disk drive, the ratio of the intensity of the light that enters the optical power monitoring detector (which will be identified herein by Pm) to that of the light that enters the storage medium through the condenser lens (which will be identified herein by Po) changes according to the optical power. As a result, the linearity of the Pm/Po ratio cannot be maintained and it is difficult to control the optical power precisely. As shown in FIG. 26, if the linearity of the Pm/Po ratio can be maintained, then the Pm/Po ratio can be represented by the line 251. On the other hand, if the linearity of the Pm/Po ratio cannot be maintained, then the Pm/Po ratio is represented by the curve 252 or the curve 253.
This is because the aperture area 232 (Area A) of the condenser lens and the photosensitive area 233 (Area B) of the optical power monitoring detector have mutually different sizes and shapes as shown in FIG. 23. And this is also because the ratio of the coupling efficiency of the laser beam that has been emitted from the semiconductor laser and has entered Area B (which will be identified by ηB) to that of the laser beam that has entered Area A (which will be identified by ηA) changes with the optical power.
That is to say, if the coupling efficiencies when the semiconductor laser has an optical power P1 are represented by ηA1 and ηB1 and the coupling efficiencies when the semiconductor laser has an optical power P2 are represented by ηA2 and ηB2, then
            η      ⁢                          ⁢      B      ⁢                          ⁢      1              η      ⁢                          ⁢      A      ⁢                          ⁢      1        ≠            η      ⁢                          ⁢      B      ⁢                          ⁢      2              η      ⁢                          ⁢      A      ⁢                          ⁢      2      is satisfied.
As a result, the linearity of the Pm/Po ratio (=ηB/ηA) is lost. In that case, the optical power of the semiconductor laser cannot be controlled precisely anymore. That is to say, the optical power of the semiconductor laser deviates from its target value.